Communication system



Jan. 13, 1942. R. J. TILLMAN COMMUNICATION SYSTEM m ESE 5&3 E

4 SheetsSheet l VENTOR By RJ T/LLMA/V 22% 6M A 7'7'ORNEV Filed Sept. 28,1940 Jan. 13, 1942. R LLMAN 2,269,565

COMMUNICATION SYSTEM Filed Sept. 28, 1940 v 4 Sheets-Sheet 2 l. INE

REC TIFIER INVENTOR By RJ TILLMA/V A TTQR/VEV Patented Jan. 13, 1942COMMUNICATION SYSTEM Richard J. Tillman, Brooklyn, N. Y., assignor toBell Telephone Laboratories,

Incorporated,

' New York, N. Y., a corporation of New York Application September 28,1940, Serial No. 358,785

25 Claims.

The invention relates to communication systems and particularly totwo-way communication systems and circuits for controlling transmissionover such systems.

The invention is particularly applicable to, although not limited to,telephone systems employing distant-talking telephone transmitters andreceivers of the loud-speaking type, as Well as amplifiers, at thesubscriber stations to enable individual telephone subscribers, or eachperson of difierent groups of persons gathered in conference at remotelylocated stations to converse with each other over an intervening linewithout the necessity of holding the receiver or being in closeproximity to the transmitter or receiver. Such systems may employ at thesubscriber stations switching circuits controlled by the voice currentsof the speaker to automatically switch loss between the transmitting andreceiving voice amplifying paths to control the direction oftransmission, to provide a desired amount of amplification and tominimize the possibility of the setting up of a singing condition due toelectrical and acoustic couplings between these paths.

An object of the invention is to improve the operation of systems of theabove-described general types.

Another object of this invention is to improve the operation of atelephone set of the general type described above and to enable it to beconnected to any telephone line in the same manner and interchangeablywith an ordinary telephone set.

A related object is to select, relatively arrange and adjust manual andautomatic devices for exercising different types of control ontransmission at a station of a two-way telephone system so as to provideimproved operation from the standpoint of quality and volume oftransmission and singing prevention with economy of apparatus and space.

Another object is to enable'two-way conversation between remotelylocated telephone subscribers or between the individuals in differentgroups of persons gathered in conference at two or more separatestations of a two-way telephone system, with a freedom of movement andfacility approaching that obtainable in face-to-face conversation ofindividuals at one location.

Another object is to enable a subscriber at one station of a two-waytelephone system to connect his telephone set With a telephone line, and

the station in accordance with his hearing requirements. I

Another object is to automatically compensate for difference in thelengths or impedance characteristics of lines connected to a telephoneset.

Another object isto automatically reduce sidetone in a telephone set.

The main objective realized by the device of the invention to bedescribed in detail below is the provision at a reasonable cost of acompact, voice-operated, distant talking, loud-speaking telephone set,preferably operated from a commercial alternating current supply, foruse with the ordinary line of a telephone subscriber, which, by notrequiring the user to hold in his hand or otherwise approach theinstrument within less than 2i feet, approximates, in case of usage, thefacility of normal direct conversation. This is accomplished with noeffect on the proper operation of the telephone circuits and little orno penalty in the grade of transmission ordinarily obtained over suchcircuits. The device realizing the above objective meets the followinggeneral specifications:

.3 1. Each component has been provided at the minimum cost which willgive the required performance.

2. The talking and listening instruments are at a suificient distancefrom the user so as to give him complete freedom of motion within alimited area.

3. The electrical volume level delivered to the telephone circuit by thetransmitter element is consistent in both average value and range withnormal telephone practice.

4. The received volume is sufiicient to enable the user to hear as wellor better than if he were using a regular telephone set.

5. Transmission quality degradation ordinarily introduced byvoice-operated switches has been minimized.

6. The transmitting circuit has beenconnected to the telephone lineterminals in such a manner as to minimize its effect on the receivingcircuit.

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' 7. The receiving circuit has been connected to the same telephone lineterminals in such manher as to minimize its effect on the transmittingcircuit.

8. Loud-speakers have been provided which will produce the desired soundlevel with a given amplifier output.

The preferred embodiment of the invention may be briefly described as acompact, economi- I cal, two-way telephone set adapted for use at tocontrol the volume of signal transmission at 59 one or more subscribersstations of a two-way telephone system, comprising a transmittingamplifying branch including a distant-talking telephone transmitter, anda receiving amplifying branch including loud-speaking telephonereceivers, coupled by a hybrid transformer and associated balancingnetwork to a two-wire line section adapted for connection to ordinarytelephone lines; and associated voice-operated switching circuits forswitching suitable losses between the transmitting and receivingbranches, to provide directional control, the desired amplification andsinging prevention, this set having the following special features:

1. To minimize the quality distortion inherent in loss switching, thefrequency-loss characteristics of the acoustic and electrical couplingsbetween the voice transmitting and receiving branches are made such asto provide minimum positive potential feedback (singing) in thefrequency band at which the maximum energy of speech is located(approximatelylOD to 1000 cycles) without affecting the frequencycharacteristics of the separate voice branches, in order to reduce theamount of loss necessary to be switched in this particular frequencyband to prevent singing, and the resistance component of each switchedloss pad is adjusted to that value which will provide adequate singingmargin with a tolerable amount of quality distortion. To accomplish this(a) The coupling between the microphone and the loud-speakers isdesigned to provide a high loss over the frequency range of maximumspeech energy. This may be obtained by a special mounting and relativearrangement of loudspeaking receivers in the voice receiving branch andthe transmitter in the voice transmitting branch, so that they areacoustically balanced with respect to each other over that frequencyrange.

(2)) The electrical coupling between the connected telephone line andthe transmitting and receiving voice branches of the telephone set isprovided by using in combination with a hybrid transformer coupling thetransmitting and receiving voice branches to the two-wire terminatingline section, of a special balancing network and a loop resistanceequalizer comprising elements having a non-linear current-resistancecharacteristic, connected in series with the terminating line section ofthe set, to provide by electrical balance relatively high magnitude andlimited variation of return loss through the hybrid transformer over thefrequency range of maximum speech energy for any one of different(length or impedance) subscribers? lines with which the set may berequired to work.

(0) A special network is included in each voice branch, having similarshaped frequency-loss characteristics over the frequency range ofmaximum speech energy (100 to 1000 cycles), the combined insertion lossversus frequency characteristic of the two networks being the inverse ofthe frequency characteristic of the singing path before insertion of thenetworks, to provide an equal margin against singing in the circularelectroacoustic loop for all frequencies in the transmission range.Provision is made for complete removal of the insertion loss of thenetwork in either voice branch by means of a voiceoperated relay undercontrol of speech energy in that branch. The distortion of the speechwaves in the voice branch caused by removal of the network is a minimumsince the insertion loss of the network is small over the frequencyrange of maximum speech energy. To enable the loss of each network to beadjusted to the proper value, it is made variable in magnitude by meansof a resistance change under control of the user of the set as acorrelative function of volume adjustment.

2. A circuit for desensitizing the receiving control of thevoice-operated switching circuit as a function of the level produced bythe talker at the input of the transmitting switching control so as toprevent false operation of the receiving switching relay when thetransmitting voice branch is operated.

3. A two-part, multistep volume control attenuator in the receivingvoice branch controlled by a multiple interlocking key switch to beoperated by the telephone subscriber or user of the set to provide therequired attenuation range of received speech volume but a smallerattenuation range of receiving control circuit sensitivity. Auxiliaryfeatures of this manual switch control are that it operates to changethe insertion loss over the frequency range of maximum speech energy ofeach of the two switched loss networks in an equal amount inversely withchange in loss of the receiving volume attenuator, and controls theconnection of the telephone set to the selected telephone subscribersloop.

Other objects and features will be brought out in the following detaileddescription of the invention in conjunction with the drawings in which:

Figs. 1 and 2, in combination, with Fig. l at the left showschematically a preferred modification of the telephone set of theinvention; and.

Figs. 3 to 8 are diagrams illustrating the principles involved and thecharacteristics of the component elements in the, telephone set of theinvention shown in Figs. 1 and 2.

The circuit of Figs. 1 and 2 includes a transmitting voice branch TC anda receiving voice branch RC coupled by the three-winding hybridtransformer HC and associated balancing network NW to the two-wireterminating section TS which is connected to the telephone line byoperation of the relay N.

The transmitting voice branch TC comprises, reading from left to right,a telephone transmitter M, an anti-cohering filter F1 which protects thetransmitter carbon from switching transients, the transmitter powersupply filter F2, the transmitting voice amplifier AT preferablycomprising two or more vacuum tube stages, and the shaped frequency-lossswitching network B1, including the variable resistance elements RT, theoutput of which is connected to a winding of the hybrid transformer H0.

The receiving voice branch RC comprises, reading from right to left, atransformer T1 having its primary winding connected across the midpointsof the line windings of hybrid transformer I-IC, functioning to providea path for the direct current fiow through the telephone line andballast lamps L1, Ia When relay N is operated, a variable resistancevolume attenuator D2 having its input connected to the secondary windingof transformer T1,-the receiving voice amplifier AR preferablycomprising two or more vacuum tube stages, a second variable resistancevolume attenuator D1 and the shaped frequency-loss switching network B2including the variable resistance elements RR, having its output coupledto a loud-speaking telephone receiver comprising two parallel-connectedloud-speakers LS1 and LS2, through the impedance matching transformerT2.

The voice-operated switching arrangement provided for switching lossesbetween the transmitting circuit TC and RC to provide directionalcontrol of the set while preventing singing and suppressing echoes andnoise, includes the transmitting switching control branch ST and areceiving switching control branch SR.

The transmitting switching branch ST includes an adjustable gainalternating current amplifier ATc consisting of two or more vacuum tubestages, having its input connected across the plate-cathode circuit ofthe last vacuum stage of the transmitting voice amplifier AT, followedby a rectifier XT, a voltage storing or hang-over network HN1, athree-electrode control vacuum tube CTT, and the transmitting switchingrelay Zr and the indicator lamp control relay Q having their operatingwindings connected in series in the cathode-anode circuit of the tubeCTT.

The receiving switching branch SR includes an adjustable gain receivingcontrol amplifier ARC consisting of one or more vacuum tube stages,

having its input connected across the platecathode circuit of the laststage of the receiving voice amplifier AR, rectifier XR connected to theoutput of the amplifier ARC through a frequency weighting network UR, avoltage storing or hangover network HNz connected to the output of therectifier Xa followed by a three-electrode control vacuum tube CTR and aswitching relay ZR having its operating winding connected to theplate-cathode circuit of tube CTR.

The voice-operated switching circuit also includes a desensitizingcircuit RCD for the receiving switching circuit SR, consisting of arectifier X1) having its input connected across the input of thetransmitting control amplifier Arc, and a voltage storing or hang-overnetwork HN3 in the output of that rectifier, connected in series withthe hang-over network HNz in the control grid circuit of the relaycontrolling tube CTR, in the receiving switching control circuit SR.

A loop resistance equalizer LE in the terminating two-wire section TS ofthe set comprises the ballast lamps L2 and L3 having suitable nonlinearresistance-current characteristics, in series with the line wires.

A manual switch G comprising a series of interlocking keys G1 to G7 isprovided for enabling the subscriber or other operator at thesubscribers station to control the energization of the telephone set,its connection to the selected telephone line, and the adjustment of theamount of resistance in the receiving attenuators D1 and D2 and in theloss networks B1 and B2 in the manner to be described below.

The telephone set as illustrated is operated from a commercialalternating current supply although it may be readily adjusted foroperation also from a direct current supply. The lowvoltage alternatingcurrent power for energizing the indicator lamp L1 and for heating thefilaments of the various vacuum tubes is obtained directly from a powersupply set P2 comprising the usual alternating current rectifier andpower transformer, supplied from a commercial alternating current line,and the power for energizing the transmitter T, the switching relay N,and for supplying the plate and grid biasing potentials for the vacuumtubes inthe amplifiers and rectifiers of the set is obtained from thatset P2 through the power supply filter F3 or that filter in combinationwith the power supply filter F2.

The transmitter M is preferably a carbon microphone of the pressuredifferential type with a bi-directional diaphragm, mounted in a baille,as disclosed in the copending application of W. D. Goodale, Jr., A.Herckmans and R. J. Tillman, Serial No. 352,692, filed August 15, 1940.This microphone M and the two parallel-connected moving coilloud-speakers LS1 and LS2 in the assembly TR, are preferably mounted andrelatively arranged in the manner disclosed in the aforementionedcopending application and illustrated diagrammatically in Fig. 7 of thisapplication, so that they are acoustically balanced with respect to eachother over the frequency range of maximum speech energy, providing ahigh coupling loss over that frequency range between the microphone andthe loudspeakers.

The small low-frequency loss network B1 in the transmitting circuit TCand the small low frequency loss network B2 in the receiving circuit RCare identical. Each may comprise the particular arrangement ofinductance, capacitance and resistance elements shown, or equivalentarrangements, with suitable relative values for these elements toprovide a frequency-loss characteristic for the network such as to givean insertion loss over the frequency range of maximum speech energywhich is low with respect to the rest of the speech range in accordancewith the acoustic and electric couplings between the voice transmittingand receiving branches. Each network is variable in resistance inaccordance with the number of units of resistance RT or RR, respectivelyconnected in the network by operation of the keys of the manual switch Gby the telephone subscriber, to provide the proper amount ofanti-singing loss depending on transmission conditions in the mannerwhich will be described later. With the particular transmission elementsused in the experimental model of this telephone set, the maximuminsertion loss provided by each network 131 or B2 was in the order of 10decibels at low frequencies.

The particular arrangement of inductance, capacitance and resistanceelements in the special balancing network NW, and their relative values,and the values of the series variable resistance (ballast) lamps L2 andL3 in the loop equalizer LE in the terminating section LS of thetelephone set, were selected with respect to the particular hybrid coilnetwork HC used, to automatically provide the required electricalbalance to give a relatively high magnitude and limited variation ofreturn loss through the hybrid coil over the frequency range of maximumspeech energy, for any telephone line with which the set may be requiredto work.

The telephone sets at other connected stations may be of the generaltype described above or may be standard telephone sets withoutvoiceoperated switching apparatus.

The operation of the telephone set of Figs. 1 and 2 will now bedescribed.

When it is desired to use this set, the telephone subscriber or otheruser automatically releases key G1 of his multiple key-controlled switchG by manual operation of any of keys G2 to G7 to cause suitableenergizing power to be supplied to the microphone M (the filaments andother electrodes of the various vacuum tubes of the set being alreadyenergized from the attenuating current supply), and to the switchingrelay N. The signal lamp L1 will then be lighted, and the switchingrelay N will operate to connect the terminating section TS of thetelephone set to a telephone line over which the connection to thedistant party or parties is established in the usual manner.

With no one talking in either direction over the set, the shapedfrequency-loss networks B1 and B2 are both effective in the transmittingcircuit TC and the receiving circuit RC, respectively, and theresistance values of these networks and that of the attenuation networksD; and D2 in the receiving circuit RC depend upon which of keys G2 to G7are operated. Whatever this operation, the loss in the electroacoustieloop provided by the set will be such as to prevent the establishment ofpositive feedback or singing. When conversation begins the subscribermay again operate any key G2 to G7 of the manual switch G :2

depending on his receiving volume requirement, which automaticallyadjusts the amount of resistance provided by the resistance elements Brand Re to control the low-frequency loss value of networks B1 and B2 toprovide the minimum amount of low-frequency switched loss consistentwith singing prevention.

When the user of the set starts to speak, his speech currents will betransmitted from the microphone T over the transmitting circuit TCthrough the anti-cohering filter F1 and will be amplified by thetransmitting amplifier AT. A portion of the amplified speech currents inthe output of that amplifier will be diverted into the transmittingswitch control Sr in which it will be divided between the input of theamplifier Arc in the main branch and the input of the receiving controldesensitizing branch RCD.

The portion of the speech signals applied to the input of amplifier Arcwill be amplified thereby and will be rectified by the half waverectifier Xr. The rectified voltage produces a variable direct currentbias on the control grid of tube CTr across the parallel resistance R1and condenser C1 of hang-over circuit HN1. this bias is initiallyadjusted by means of the adiustable gain control of amplifier Arc tocause the operation of the transmitting switching relay Zr by the platecurrent of tube Cr at a predetermined speech level. The polarity of thisvariable bias is in opposition to the fixed bias on the control grid oftube CTr which ordinarily permits only a slight current to flow throughthe tube and the windings of the transmitting switching relay Zr and theindicator control relay Q in series in its output which is notsufficient to operate these relays. As the variable bias changes inmagnitude due to increasing speech level at the input of the controlamplifier Are, the increasing flow of current through the relay controltube Crc and the winding of switching relays Zr and Q causes relay Zr tooperate to close its relay contacts I, 2 and 3, and relay Q to operateto short-circuit indicator lamp L1, extinguishing it and thus indicatingto the The magnitude of talker that the transmitting switching controlSr is operating properly.

The closing of contacts l of relay Zr effectively short'circuits theshaped frequency-loss network B1 in the transmitting voice path TC,allowing the main portion of amplified speech energy in the output oftransmitting amplifier Ar, to be transmitted over the outgoing portionof transmitting circuit TC through hybrid coil HC and loop equalizer LEto the connected telephone line with little additional attenuation. Theclosing of contacts 2 of relay Zr disables the receiving amplifier AR,in. receiving circuit RC by connecting one side of its input line toground. The closing of relay contacts 3 operates to bring the potentialof the control grid of the amplifier Anc to ground, thus disabling thereceiver control circuit Sn in its input. The sensitivity and thereforethe time constants of the relay contacts l 2 and 3 of the transmittingswitching relay Zr are made such that the latter two functions areperformed before the removal of the loss 131 from the transmitting voicepath TC.

When the subscriber at the station ceases talking or the level of hisspeech currents at the input to the transmitting control circuit Srdecreases belcw a given minimum value, the variable bias applied to thecontrol tube CT'r in the circuit Sr is maintained sufficiently high tocontinue the closure of the contacts of relay Zr for a desired hang-overtime interval determined by the values of the resistance and capacitanceelements R1 and C1 in the hang-over network HM, and when relay Zrreleases the time constants of the relay contacts are such as to causethe network B1 to be reintroduced into the transmitting circuit TCbefore the receiving circuit RC is again enabled.

The use of the desensitizing circuit RCD was I found to be necessarybecause of the use of the small low-frequency switched loss pads whichpermit the part of the transmitted speech energy passing through thehybrid coil EC to the input of the receiving switching control SR, dueto residual unbalance, to be at a rather high level, higher for instancethan the lowest line level at which this control is supposed to operate.Since the transmitting switching control Sr, which disables thereceiving control Sn, cannot be made too sensitive if it is not to beoperated by room noise, the unbalance currents passing into thereceiving circuit RC may cause the receiving control SR to falselyoperate to disable the transmitting side when the set is being used fortransmitting. The circuit RCD prevents this by causing the re ceivingcontrol SR to be desensitized in proportion to the talking level at theinput of the transmitting control Sr so as to prevent operation of thereceiving switching control Sn when the transmitting circuit TC istransmitting signals. The time constants of the circuit elements in therectifier and associated hang-over network of the circuit RCD areselected so that the circuit has a fast attack time, faster than that ofthe receiving switching control and less hold-over The circuit RCDproduced on the control grid of that tube by hang-over network HNz inresponse to the output of rectifier Xo, varying as a function of thespeech level at the input of the receiving control SR. The fixed bias onthe relay control tube CTR which must be overcome to produce sufiicientcurrent flow to the winding of relay ZR to operate it, represents marginagainst operation of the relay contacts.

The portion of the talkers speech currents diverted to the desensitizingcircuit RCD is rectified by the half Wave rectifier Xi) and therectified voltage varying in accordance with the talkers speech currentsis applied across the resistance R8 and parallel condenser C8 of thehangover network HNx to produce a third bias on the control grid of tubeCTR. This third (variable) bias is in series with the other two biases,and is poled so as to aid the fixed bias and oppose the other variablebias supplied by hang-over network HN2. By proper selection of thevalues of the elements in the desensitizing circuit RCD, the magnitudeof this third bias produced by the desensitizing circuit RCD on thecontrol grid of the tube CTR is made equal to the variable bias appliedthereto from the receiving control SR which would ordinarily cause falseoperation due to speech levels from the transmitter passing through theunbalance of the hybrid coil for the case where the switch G is operatedto produce high received speech volume. As a result, the opposingvarying biases cancel one another, leaving the margin against operationof switching control SR originally provided by the normal fixed bias soas to maintain the receiving switching relay ZR unoperated while thelocal subscriber is talking. The desensitizing circuit makes it possibleto maintain suitable operation of the terminal as a whole Witha smallerlowfrequency switched loss than would otherwise be and a consequent lossin intelligibility. A reduction in the magnitude of the switched lossover this range reduces such undesirable effects. The use of the circuitRCD also makes it possible to use a higher sensitivity for the receivingswitch- 1 ing control SR so as to enable its operation on lower levelsignals incoming from the line.

The portion of the talkers voice energy transmitted from the output ofthe transmitting voice circuit TC to the input of the receiving voicecircuit RC, due to any unbalance in the hybrid coil HC and associatedbalancing network NW, will not reach the loud-speaking receivers LS1 andLS2 because of the previous blocking of the receiving voice amplifier ARby operation of the transmitting switching control ST in the mannerwhich has been described.

Now, let it be assumed that when the local subscriber ceases talking orpauses for a sumcient time interval to release the transmittingswitching control ST speech currents are received over the connectedtelephone line from a distant telephone subscriber and pass through thehybrid transformer 1-10 to the receiving voice branch RC.

The resistance of the non-linear ballast lamps L1 and L2 in the loopequalizer LE which varies directly with the relatively high directcurrent flowing through a telephone loop but is not affected by therelatively weaker speech currents, becomes stable at a value determinedby the electrical length of a given loop and the associated telephoneline battery (not shown) the instant the set is connected to a telephoneloop and thus automatically adjusts the balance of the hybrid coil setso as to compensate for differences in the length of the connectedsubscribers line from the average length line for which the compromisebalancing network NW has been designed, so as to maintain the returnloss through the hybrid transformer I-IC high and substantially constantover the frequency range of maximum speech energy. This automaticbalance, then, is present for any condition of transmitting or receivingand is independent of speech currents.

An alternative use for the combination of the hybrid transformerbalancing network and equalizing ballast lamps is to automaticallyreduce the volume of side-tone in the telephone set.

The received voice currents will be transmitted through and attenuatedin the volume attenuator D2 except in the maximum volume condition and,the receiving circuit RC being unblocked due to the unoperated conditionof the transmitting switching control Sr, will be amplified in thereceiving speech amplifier AR. The amplified voice currents will then bedivided between the input of the receiving volume attenuator D1 in thereceiving voice branch RC and the input of the receiving switchingcontrol SR.

The portion of the received voice currents transmitted into thereceiving switching control SR will be impressed on the input of thevariable gain amplifier Arm and will be amplified thereby in accordancewith its gain setting. The amplified waves in the output of theamplifier ARc will then pass through the frequency weighting or shapingnetwork UR to the rectifier XR. The network UR comprises a transformerT3, which transformer is tuned by the condensers C4 and C5 connectedacross its primary and secondary windings, respectively, to a frequencyapproximately in that portion of the speech range of highest power, thatis, from to 1000 cycles. This tuning is rendered rather fiat by theresistance elements in the circuit, such as resistance R5. The circuitthus discriminates against the extreme low and high frequencies ofcircuit noises in the telephone band of transmission and produces afavorable signal-to-noise ratio for the switching control SR for thehigher energy telephone speech frequencies. Additional low frequencydiscrimination is attained by the use of the series condenser C7.

The speech waves in the output of the shaping network UR are rectifiedby the rectifier circuit XR consisting of a half-wave rectifier tube,the resistance R6 and parallel condenser C6. The direct current voltagethereby developed across the resistance Re, the desensitizer circuit RCDbeing unoperated due to the absence of applied transmitted speech anddue to the large coupling loss between loud-speaker and transmitter,serves to overcome the fixed bias on the control grid of the receivingcontrol tube CTR causing operation of the receiving switching relay ZRby the tube output current.

. The operation of relay ZR to close relay contacts 4 effectivelyshort-circuits the shaped frequency-loss network B2 thereby removing theanti-singing loss from the voice receiving branch RC. The closing ofswitch contacts 5 efiectively disables the voice transmitting side TC ofthe set by connecting the control grid of the last vacuum tube stage ofthe transmitting amplifier AT to ground. The closing of the switchcontacts 6 connects the control grid of the last tube of thetransmitting control amplifier Arc to ground, thus disablingthetransmitting switching control ST; contacts and 6 functioning beforecontact 4.

The amplified speech energy in the output of the receiving amplifier ARwill be transmitted through and attenuated in volume attenuator D1except on the higher volume conditions and, the shaped frequency-lossnetworkB2 having been effectively eliminated, will then be transmittedwith little attenuation through transformer T2 to the loud-speakers LS1and LS2 and will be heard by the local subscriber. The local subscriber,then, by depressing the keys G2 to G7 consecutively :may add anincreasing number of units 'of resistance first in the volume attenuatorD2 and then in the volume attenuator D1 to reduce the acoustic level ofthe received speech, or by depressing the same keys in reverse order mayprogressively cut out resistance units first in volume attenuator D1 andthen in volume attenuator D2 to increase the acoustic level of thereceived speech, in accordance with his hearing requirements and thelevel of incoming speech on the telephone .line. This arrangementprovides in addition to 'therequired attenuation range of the receivedspeech volume, a smaller attenuation range of sensitivity of thereceiving switch control SR, due to the fact that only the Volumeattenuator D2 is effective in varying the level delivered to thereceiving switching control whereas both volume attenuators D1 and D2are effective in the receiving voice branch RC. This limitation ofreduction in sensitivity of the re-; ceiving switching control SR. asthe received volume is reduced, provides higher sensitivity of thecontrol circuit with respect to high incoming speech levels giving abetter performance of the set on the higher speech levels which willprobably be typical of the majority of calls.

The operation of the keys G2 to G7 by the local subscriber ostensibly tocontrol volume provides in addition to a variation of the loss in thevoice receiving circuit RC to provide proper control of the receivingspeech volume and receiving switching control sensitivity, an inversecontrol of the amount of loss in the frequency range of maximum speechenergy in the frequency-loss networks B1 and B2. gain is not needed to'provide adequate levels as is the case with the majority of calls, lessloss may be switched with a resulting improvement in speechintelligibility. Therefore when the keys Gz to G1 are operated to stepdown the receiving volume (by increasing the amount of loss in thereceiving voice path) they simultaneously operate to control the numberof resistance units in the variable resistance portions ventionillustrated in' Figs. 1 and2 and described above.

Fig. 3 shows the'required gains and losses in the two directions oftransmission and the loudspeaker transmitter coupling loss and returnloss 'through'the' hybrid coil in'the form of a level diagram obtainedwith an experimental model of If the maximum available upwards to theears of the user.

the set. The heavy solid line from point N to point 0 shows that thegain which must be provided in the transmitting voice branch is afunction of the required line level at point 0, when a talker speaksjust loudly enough above average room noise at point N to operate thetransmitting switching control. The heavy line from point P to point Qillustrates the corresponding requirements for the receiving voicebranch. In this case, the predetermined speech level depends on averageroom noise sound level, and sufficient gain must be available to deliverthis speech level for the lowest incoming level from the telephone lineat which the receiving switching control candiscriminatebetweenspeechandaveragelinenoise. The two dashed lines whichstart from the output of the transmit amplifier and depart from thesolid lines are for the case where both transmit and received switchedlosses are left in the circuit and for the case where both are omitted.These lines illustrate the need for adding losses to obtain a positivesinging margin.

The diagrams of Fig. 4 show a comparison of the change in speech energyversus frequency caused by the insertion or removal of a loss shapedwith frequency, as in the system of the invention, or of a loss constantwith frequency. Where the singing path is so designed as to have minimumgain in the region of maximum speech energy and the switched loss isshaped accordingly, it will be seen from these diagrams that there isless change in the range of maximum speech energy than if a lossconstant with frequency, whose magnitude is determined by the singingpath gain at the singing frequency, is switched in and out of thecircuit. Since the energy content of the speech is relatively less atthe higherfrequencies, the absolute magnitude of any transient inducedby switching at those frequencies might be expected to be such as tocause little disturbance. It should also be pointed out that abruptswitching of loss at the lower frequencies has an annoying effect on thelistener because of the change in loudness of the background room noise,and that this effect is minimized by restricting the magnitude of theloss switched at these frequencies.

The diagrams of Fig. '7 illustrate the functioning of the particulararrangement provided for reducing the acoustic coupling between thetransmitting and receiving voice circuits in the system of theinvention. As indicated, two moving coil loud-speakers and a carbontransmitter of the pressure difierential type (with diaphragm accessibleto sounds from front and rear) mounted in abafile comprise thisarrangement. The moving coils of the two loud-speakers are connected inparallel in such a Way as to have the cones in phase as regards motiontowards and away from the center line. The loudspeakers aresymmetrically mounted with respect to the transmitter, and the axisthrough the center of the cone of each loud-speaker lies in the sameplane as the axis perpendicular to the transmitter diaphragm which isalso its axis of maximum efficiency for speech. In an experimental modelof this arrangement which has been built and tested, the whole assembly(TR) is contained in a small cabinet with the plane of the face of eachloud-speaker inclined at such an angle as to direct the sound from desklevel The baffle con taining the transmitter is mounted at the top ofthe cabinet. The cabinet completely encloses the loud-speakers andcontains sound-absorbing material so that the loud-speakers radiate onlyfrom the outer surfaces of the cones. The

-method of acoustic balance is illustrated at the right of this figure.It is shown that due to the equal distance and phase of theloud-speakers with respect to the transmitter, no net pressure isdeveloped on the transmitter diaphragm by the direct action of theloud-speakers. There is a variation in the net pressure developed withincreasing frequency such that the net pressure on the transmitterdiaphragm, while it is zero or low at low frequencies, increases to someextent with increasing frequency, giving a relative coupling loss of theshape shown at the left diagram of Fig. 5. As regards speech directed atthe transmitter, there is a net pressure exerted on the diaphragm due tothe difierence in'pressure on the front and rear. This net pressuredecreases, of course, as the speaker moves off the central axis of thetransmitter. There is a corresponding discrimination afforded againstroom noise of random incidence due to this pressure differentialcharacteristic. It should be pointed out that the high coupling lossover the range of maximum speech energy frequencies as provided by thistype of balance is desirable for the prevention of false operation ofthe transmitting switching control circuit due to direct sounds from theloud-speakers.

The diagrams of Fig. 8 illustrate the function of the loop resistanceequalizer (LE in Fig, 2). The action of the variable resistance ballastlamps (L2 and L3) in this equalizer is to limit the range of loopresistances which might ordinarily be expected in practice to beconnected to the set. At the expense of a small transmission loss addedto the loss of a short loop, the equalizer by adding resistance to thegiven loop insures that in connection with the balancing network(element NW in Fig. 2), the return loss through the hydrid coil (elementH0 in Fig. 2) for any of the multiplicity'of impedances connected to thecentral office end of the loop will be high over the range of maximumspeech energy and of nearly the same magnitude as with a long loop. Ofcourse, the transmission loss due to ballast lamp resistance is smallerfor a long loop since the lamp resistance is a function of the currentpassing through it. As shown in Fig. 8, there is, with a balancingnetwork designed specifically for the frequency range in question (suchas NW in Fig. 2), a particular value of resistance between the telephoneset and the varied impedance characteristics of the central oince end ofthe telephone connection, which gives the optimum loss for allfrequencies in the given range. In the curves to the right in Fig. 8 thecomplementary relationship between the loop resistance and lampresistance as a function of current is shown. It is evident that theloop plus the ballast lamp resistance for any given loop will be held toa limited range.

The diagrams of Fig. 6 illustrate the function of the control switch (Gin Fi 1) which varies both the receiving volume and the switchingnetwork losses. As shown in the diagram to the left of the figure, thepotential negative singing margin (the net gain in the closedelectroacoustic loop without switched loss) varies with the amount ofreceiving volume attenuation necessary for proper reception in a givenconversation. The low frequency loss portion of the two switched lossnetworks, as shown by the center diagram of Fig. 6, varies inverselywith the receiving volume attenuator loss. As described above, this isaccomplished by varying the amount of the resistance RT and RR in thenetworks B1 and 132, respectively, by suitable operation of the commonswitch control G. The latter networks are symmetrical and theirresistance values are variable in an equal amount. The effect of thisvariation is to decrease the energy change due to switching at thefrequencies of maximum speech energy, as the receiving 3 speech isdecreased for calls having higher incoming level. As indicated in thediagram at the right of Fig. 6, the switched losses are decreased insmaller increments than the receiving attenuator losses are increased.The differential provided is a compromise between the desirability ofdecreasing the switched loss as the receiving attenuator is varied forminimum attenuation, and the provision of some additional singing marginfor the stronger volume calls which are less apt to provide smoothimpedances at the near-end central office, because there is usually lesstrunk involved. It will be noted that the singing margin is greater atthe higher frequencies for any condition of the common control G,

and that the increment of additional singing margin added at thesefrequencies is equal to the increment of loss inserted by the receivingvolume attenuators. This results because the shaped loss networks B1 andB2 are simpler in construction if only the low frequency loss is changedby varying the amount of resistance in the resistance element, becausethe switching of loss at the high frequencies is believed to producelittle distortion and also since the return loss is not controlled atthese frequencies.

A telephone set having circuit elements substantially as disclosed inFigs. 1 and 2 and described above, has been built and tested. Each itemof apparatus on this set was selected with the ultimate cost in View andthere is none which is inherently expensive. The set consists of twoparts, one a small desk mounting containing the microphone,loud-speakers and volume control, and the other apparatus beingcontained in a cabinet approximately 1 cubic foot in volume, which maybe located at some distance from the desk set. The tests which have beenmade on this set show that it provides transmission substantiallyequivalent to that provided by the ordinary telephone set and thereforeis not limited as to circuit length. The talking and listening distancesare such that convenience of conversation and freedom of motion areprovided, and satisfactory operation is maintained over the range ofnoise levels that are likely to be encountered in private offices.

A later model of the telephone set which has been constructed and testeddiifers from that which has been illustrated and. described mainly inthe addition of a control circuit expander and the use of negativefeedback on the control amplifier, in the transmitting switching controlST, as disclosed in the copending application of H. W. Augustadt, SerialNo. 351,869, filed August 8, 1940. The use of the shunt feedback on thecontrol circuit amplifier decreases the attack time of the transmittingswitching relays somewhat and the use of the expander provides a bettersignal-to-noise ratio in cases where room noise level is high andreduces the range of input signal levels over which the hang-over timevaries with input level, these additions improving the operation of theset to some extent.

Another feature which may be incorporated in the telephone set of theinvention is the use of negative feedback in connection with the voiceamplifier in the transmitting voice path or in the receiving voice pathor both, to stabilize its voltage output so as to provide in addition toimprovement from the standpoint of reducing distortion in thetransmitted voice signals, the prevention of variation in the input ofthe associated voice operated switching circuit with variation in? theimpedance of the load circuit connected to the voice path with removalof loss from that path by the switching circuit, thereby decreasing thecharge of false operation or singing, as disclosed in the copendingpatent application of H. W. Augustadt, Serial No. 285,042, filed July18, 1939, which issued as Patent 2,214,80 on Sept. 17, 1940.

Although the telephone set of the invention wasv primarily devised foruse on the telephone subscribers premises as a desk set, it is apparentthat it is applicable as well for use in telephone pay stations in whichcase the microphone, loudspeakers and control means arranged asdescribed above, would be disposed in a telephone booth in a suitablemanner and the amplifier and auxiliary equipment mounted inside oroutside the booth and connected to a telephone line. Preferably,suitable means would be provided for controlling the acoustic conditionsinside the booth, and the set would be combined with a coin collector orany other standard equipment required to provide the telephone service,in any suitable manner.

The devices described herein may also be used in connection with othertypes of instruments and circuits in which a means is required for thesuppression of side-tone or singing. For example, they are suitable foruse in universal telephone line service, announcing, intercommunicationor other direct wire service, or in radio telephone service. They may beused with any type of two-way transmission medium, such as two andfour-wire telephone circuits. Other modifications of the circuitsillustrated and described which are within the spirit and scope of theinvention will occur to persons skilled in the art.

What is claimed is:

1. A station for a two-way telephone system including a plurality ofstations connected by a wave transmission medium, comprising two circuitbranches each including an electroacoustic transducer operatingrespectively as a telephone transmitter and a telephone receiver,connected to said medium, voice-operated control circuits respectivelycontrolled from a different one of said branch circuits for controllingthe transmission efliciency or said branch circuits in such manner as toprovide directional control of voice signal transmission at the station,and means to minimize. voice signal quality impairment due to thechanges in transmission eniciency of said branch circuits in providingthe directional control comprising means to make the transmissioneiiiciency changes relatively smaller over the frequency range ofmaximum speech energy than over the remainder of the speech frequencyrange.

2. The station of claim 1, in which the lastmentioned means provides therecited result without impeding transmission over the circuit branchtransmitting signals.

3. The station of claim 1, in which said voiceoperated control circuitsprovide the desired directional control by switching out a normalantisinging lossin the branch circuit transmitting signals and disablingthe other branch circuit, and the last-mentioned means comprises meansfor making the loss-frequency characteristics of the anti-singing lcssswitched such that; it normally provides an insertion loss which isrelatively less over the frequency range of maximum speech energy thanover the rest of the speech frequency range.

4. A two-way communication system comprising a plurality of stationsconnected by a twoway transmission medium, at least one of said stationscomprising two circuit branches each terminated in an electroacoustictransducer respectively operating as a signal transmitter and areceiver, and connected to said medium, and means to reduce the amountof disturbing energy transmitted between the two transducers over saidcircuit branches comprising a common cou pling means between saidcircuit branches and said medium providing a high loss between thebranches over the frequency range of maximum energy in the communicationsignals.

5. The system of claim 4, in which said common coupling means comprisinga hybrid transformer, an associated line balancing network andautomatically variable resistance elements providing in combination therequired high loss between said branches over the frequency range ofmaximum signal energy.

6. The system of claim 4, in which said common coupling means comprisesa hybrid transformer coupling said branches to each other and to saidtransmission medium, an associated line balancing network and anautomatic line-length equalizing device of such relative characteristicsas to provide between said circuit branches the desired high loss oversaid frequency range of maximum energy in the communication signals,enabling a relatively smaller change to be made in the transmissionefficiency of a circuit branch over that frequency range where anincrease in ehiciency of that branch is required in signal transmission.

7. The station of claim 1, in which said transmission medium is atwo-way transmission line and said two circuit branches are coupled incommon to said line.

8. The station of claim 1, in which said twoway transmission mediumcomprises two oppositely-directed transmission circuits, thetransmitting circuit branch being connected to the one of the lattercircuits transmitting in the same direction, and the receiving circuitbranch being connected to the one of the latter circuits transmitting inthe opposite direction.

9. The station of claim 1, in which said voice signal quality impairmentis further minimized by auxiliary means for simultaneously reducing themagnitude of a change in transmission efficiency of one or both circuitbranches in the frequency range of maximum speech energy produced byoperation of a voice-operated control circuit.

101A station for a two-way telephone transmission system including aplurality of stations connected by a wave transmission medium,comprising a transmitting circuit branch including a. telephonetransmitter, and a receiving circuit branch'including a receiver,connected to said medium, voice operated control circuits forcontrolling the transmission eiiiciencies of the two circuit branches toprovide directional control of signal transmission therein and auxiliarymeans to provide optimum relative efficiency of said circuit branches toimprove the quality of speech signal transmission over the systemcomprising a common control for effecting a desired magnitude of changein the transmitting efficiencies of one Or both circuit branches withchange of the efiiciencjies of said voice-operated control circuits.

11. A telephone set comprising a voice transmitting amplifying branchincluding a distanttalking telephone transmitter, a voice receivingamplifying branch including a loud-speaking telephone receiver, atwo-way terminating circuit adapted for connection to a selectedtelephone line, common means for electrically coupling said transmittingbranch and said receiving branch in transmission relation with saidterminating circuit, means providing a normal loss in each of said voicebranches sufiicient to prevent singing in said set in the absence ofVoice signal transmission over either branch, a voiceoperated switchingdevice connected to each voice branch, responsive to voice signaltransmission in the connected branch, when it is operative, to switchloss out of that branch to provide the desired transmission gain, and todisable the other voice branch and the switching device connectedthereto in order to directionally control signal transmission, suppressechoes and prevent singing and false switching, and means to improve theoperation of said set comprising means to make the frequency-losscharacteristics of the losses switched, and of the acoustic andelectrical couplings between said transmitting voice branch and saidreceiving voice branch such as to reduce to a low value positivepotential feedback at the frequencies of maximum speech energy in theelectroacoustic singing loop formed by said voice transmitting and voicereceiving branches and the couplings therebetween, without affecting thefrequency characteristics of the separate voice branches.

12. A telephone set comprising a voice transmitting branch including adistant-talking telephone transmitter and a one-Way amplifier, a voicereceiving branch including a loud-speaking telephone receiver and aone-way amplifier, a two-way terminating circuit adapted for connectionto a selected telephone line, common means electrically coupling saidvoice transmitting branch and said voice receiving branch to saidterminating circuit, loss networks respectively connected in said voicetransmitting branch and said voice receiving branch, which, in theabsence of voice transmission over either branch, provide equal lossestherein suflicient to prevent singing around the electroacoustic loopformed by the two Voice branches, voice-operated switching circuitsrespectively connected to said voice transmitting and said voicereceiving branch, responsive to voice signal transmission in theconnected voice branch, when it is operative, to switch out the lossnetwork in that branch to provide the desired transmission gain, and todisable the other voice branch and the switching circuit connectedthereto to control the direction of transmission and to prevent singingand false switching, and means to improve the operation of the set fromthe stand-point of transmission quality and singing prevention,comprising means to make the acoustic coupling and said electricalcoupling between the voice transmitting and receiving branchesrespectively provide a large transmission loss in the frequency range ofmaximum speech energy, and means to adjust the loss in said frequencyrange of said loss networks to the minimum values which will provideadequate singing margin with quality distortion within a tolerablevalue.

13. The telephone set of claim 11, in which the desired characteristicsare attained by employing in each voice branch a network with a shapedfrequency-los characteristic providing an insertion loss over thefrequency range of maximum speech energy low with respect to the rest ofthe voice frequency range, and variable in magnitude by means of aresistance change, the loss network in each voice branch beingeffectively removed by operation of the voice-operated switching deviceconnected to that voice branch, the maximum amount of loss provided byeach loss networkat low frequencies being in the order of ten decibelsor less, and the resistance of each loss network being adjusted to thevalue which will provide adequate singing margin with a tolerable amountof quality distortion, and by employing in said electrical coupling ahybrid transformer with an associated line balancing network and a loopequalizer in said terminating circuit, providing in combination arelatively high magnitude and limited variation of return loss throughsaid hybrid-transformer between said voice transmitting and receivingbranches over the frequencyrange of maximum speech energy for anytelephone line on which the set may be required to work.

14. The telephone set of claim 12 in which said loss networks provide asmaller loss and said acoustic and electrical couplings a larger loss inthe frequency range of to 1000 cycles than they do to frequenciesoutside that range.

15. The system of claim 12 in which said common electrical couplingcomprises a balanced bridge consisting of a hybrid coil and associatedbalancing network, said terminating circuit in, cludes an equalizer ofseries elements having a non-linear current-resistance characteristic,the frequency-loss characteristics of said balancing network beingselected to balance the average telephone line connected tosaidterminating circuit, and the varying resistance of said serieselements with direct current from the line being such as to adjust thebalance of saidbridge, so as to adjust the balance of said bridge, so asto insure a high magnitude and limited variation of return loss throughsaid hybrid coil between said voice transmitting and voice receivingbranches over said frequency range of maximum speech energy for any oneof a range of telephone lines to which the set may be connected. 16. Thetelephone set of claim 12 in which the voice-operated switching circuitconnected to the voice transmitting branch includes a portion responsiveto the applied voice energy to reduce the sensitivity of thevoice-operated switching circuit connected to the receiving voicebranch, in proportion to the increase in amplitude level of said appliedvoice energy, so as to prevent false operation of the latter switchingcircuit by signal echoes of a level higher than the lowest signal levelsat which that switching circuit is designed to operate, while said voicetransmitting branch is transmitting signals.

17. In combination with a two-way telephone system includingoppositely-directed, one-way voice transmission paths and voice-operatedswitching circuits each including an amplifier preceded by a rectifierconnected to each oneway path, operatively responsive to appliedtelephone signals to increase the transmission efficiency of that pathand to disable the other voice path and the connected switching circuit,an auxiliary control circuit operating to reduce the sensitivity of theamplifier in at least one switching circuit as a function of the signalamplitude level produced by a talker at the input of the other switchingcircuit, in order to prevent false operation of said one switchingcircuit by speech energy transmitted between said paths.

18. The telephone set of claim 11 in which said voice receiving branchincludes volume control means comprising two or more variableattenuators so located and controlled as to produce a desiredattenuation range of received speech signals, but a smaller attenuationrange of sensitivity of the switching device connected to said voicereceiving branch, providing improved performance of the set for higherlevel received signals.

19. In combination in a two-way telephone system, two oppositelydirected one-way signal transmission paths, a voice-operated switchingdevice connected to each path, responsive to voice signal transmissiontherein, in the absence of prior voice signal transmission in the otherpath, to increase the transmitting efiiciency of the first path and todisable said other path, so as to directionally control signaltransmission in said system while suppressing echoes and preventingsinging, volume control means including two or more variable attenuatorsin at least one of said signal transmission paths, so located andcontrolled as to produce a desired attenuation range of transmittedspeech signals in that path and a smaller. attenuation range ofsensitivity of the connected voice-operated device.

20. A telephone set in accordance with claim 12, in combination withmanual means operated by the local subscriber for varying theattenuation in the voice receiving branch to control the acoustic volumelevel produced at the loudspeaking receiver, and for producingsimultaneously an equal change in the resistance values of the two lossnetworks in the voice transmitting and receiving branches which is theinverse of the change in attenuation produced in the voice receivingbranch.

21. A telephone set in accordance with claim 12 in combination with atwo-part multi-step volume control attenuator in the receiving voice Lbranch one part on each side of the receiving switching circuit input,and an interlocking key manual switch controlled by the local telephonesubscriber for simultaneously controlling the adjustment of saidattenuator so as to produce a desired attenuation range of receivedsignal volume and a smaller attenuation range of receiving switchingcircuit sensitivity and the adjustment of the resistance of said lossnetworks to produce an equal change of their insertion loss which overthe frequency range of maximum speech energy is the inverse of thechange in the receiving loss produced by the adjustment of said volumecontrol attenuator.

, 22. The telephone set of claim 12 in combination with an interlockingkey switch controlled by the local operator, for controlling theenergization of said set, connecting it to the selected telephone line,and controlling the received volume level, the sensitivity of thereceiving voiceoperated switching circuit and the loss value of the losnetworks in the voice transmitting and receiving branches.

23. The combination of claim 17, in which said auxiliary control circuitcomprises means responsive to said talkers signal waves at the input ofsaid other switching circuit to apply a direct current bias varying inaccordance with the level of said waves to the amplifier in said oneswitching circuit poled to oppose the varying direct current biasproduced thereon by transmission applied to the input of said oneswitching circuit, and of such magnitude as to prevent operation of saidone switching circuit in response to echoes of said talkers signal wavesof higher level than the minimum level at which said one switchingcircuit is supposed to operate.

24. The combination of claim 17, in which said auxiliary control circuitcomprises means responsive to said talkers signal waves at the input ofsaid other switching circuit to apply a direct current bias varying inaccordance with the level of said waves to the amplifier in said oneswitching circuit poled to oppose the varying direct current biasproduced thereon by transmission applied to the input of said oneswitching circuit, and of such magnitude as to prevent operation of saidone switching circuit in response to echoes of said talkers signal wavesof higher level than the minimum level at which said one switchingcircult is supposed to operate, the circuit elements in said auxiliarycontrol means having such characteristics with respect to those of thecircuit elements in said one switching circuit as to give said auxiliarycontrol means a faster attack time and a smaller holdover time than saidone switching circuit.

25. The combination of claim 17, in which each of said switchingcircuits includes a rectifier followed by an electron dischargeamplifying device having a control grid circuit including a hangovernetwork, and a switching relay controlled by the output current of saiddevice, said device having a normal fixed direct current biasSllfilcient to prevent operation of said switching relay in response tonormal circuit noise applied to said network but permitting operation ofsaid relay from the output of the preceding amplifying device inresponse to waves of greater level applied to said one switchingcircuit, and said auxiliary control circuit comprises a half-waverectifier supplied with waves applied to the input of said otherswitching circuit, and a second hang-over network in the output thereofand connected in series with the first hang-over network in the controlgrid circuit of the electron discharge device in said one switchingcircuit, the values of the circuit elements and the poling of saidhalf-wave rectifier in said auxiliary control circuit being selected sothat the direct current biasing voltage varying in accordance with thewaves in the input of said other switching circuit, applied by saidsecond hang-over network opposes that applied by said first hang-overnetwork, and is of sufiicient magnitud to prevent operation of saidswitching relay in said one switching circuit in response to the talkerssig nal echoes reaching the associated voice transmission path.

RICHARD J. TILLMAN.

